1. Field of the Invention
The invention relates to a method of transmitting or storing a digitized encoded audio signal composed of a sequence of information blocks over interference affected channels, wherein the individual information blocks are composed of information units containing different informations, such as control informations, scale factor informations as well as informations about the encoded sampled values derived from a sub-band and/or transformation source coding process, with the units for the informations being associated with respect to the encoded sampled values with a certain spectral structure and wherein the encoded audio signal is subjected to channel coding that is dimensioned according to the desired error protection, with an error correction being effected in the case of the detection of an error and, if such errors are not correctable, the errors being concealed. Such a method is disclosed in International Publication WO A, 92 179 48.
2. Description of the Related Art
For the transmission of digitally coded audio signals over radio channels or other interference affected channels it is desired to have a relatively high bit error protection at the transmitting end in order to ensure reception under poor transmission conditions as well, for example, when there is shading in mobile reception. This requires a correspondingly high redundancy in the transmitted binary positions of the digitally coded signal which is connected with high data rates. To reduce the data rate, bit saving source codes based on a reduction of redundance and irrelevance are usually employed before the channel coding. In such bit saving source coding methods, the digitally sampled audio signal is converted into sub-band sampled values by using a filter bank and/or into spectral sampled values by using a transformation into the frequency domain. In addition to the sampled values, additional and control informations, such as, for example, scale factors, scale factor selection informations and bit assignment informations are transmitted, that is, successive information units of different content must be transmitted, with a sequence of such information units being combined into an information block (also called "information frame"). Bit errors at individual binary positions within an information block produce a different, subjective noise effect for the listener. For example, the interference in a bit assignment in the radio channel may temporarily lead to a complete absence on the playback side and thus to muting of the audio signal for a certain duration. These control informations must therefore be protected to a considerably greater extent than the encoded sampled values. In connection with the error protection of the encoded sampled values, it applies that the listener has difficulties discerning a bit error in a lower value binary position of the encoded sampled value and sometimes is unable to discern it at all.
DE 3,805,169.C2 discloses the provision of a variable bit error protection which is adapted to the varying significance of the individual binary positions of each information block. In the case of increasingly poorer transmission conditions, the less important binary positions of the information block are interfered with first since these binary positions receive less error protection than the higher valued binary positions. With increasingly poorer transmission conditions, the interference progresses to increasingly more important binary positions, with the subjective noise effect being describable as "discernible", "slightly annoying" up to "unpleasantly annoying". In this connection it is assumed that it is possible only to correct the error in the received encoded audio signal data not to conceal the errors so that relatively high subjective noise effects may develop.
From the International Publication WO, A, 92 179 48 it is known to positively conceal interfered-with sub-bands or spectral values and thus reduce the subjective noise effect.